Welding of Ferrite-Austenitic Duplex Stainless Steel​

- Mar 28, 2018-

Ferritic-austenitic duplex stainless steel is a kind of duplex stainless steel developed on the basis of ultra-low carbon ferritic stainless steel. It is a dual-phase structure at room temperature. Compared with general stainless steel, the mass fraction of Ni is low, but the mass fraction of Cr and N is high, and it has better resistance to pitting and stress corrosion. In addition, its crystalline structure has a high mass fraction of Fe and therefore has a higher yield strength than other stainless steels.


Ferritic-Austenitic Duplex Stainless Steel Welding Features Ferritic-Austenitic duplex stainless steels have good weldability. Ferritic-Austenitic duplex stainless steels are heated to a sufficient temperature. Body to ferrite transformation. As the temperature increases, ferrite increases and austenite decreases. When the temperature rises to 1250<1300>, it can be transformed into pure ferrite. At this point, cooling is again performed, and pure ferrite structure can be obtained at room temperature. When cooled from the liquid phase, it begins to solidify at 1 450 >ferrite. When the temperature is lower than 1300 >, crystal nuclei are first formed at the ferrite grain boundaries, and austenite is gradually formed. The slower the cooling rate, the more austenite is generated, and the less austenite is produced. The duplex stainless steel is less prone to cracking in welding than ferritic stainless steel and has a lower tendency to embrittle after welding than austenitic stainless steel.


However, if the welding process is not well mastered, the dual phase structure of this material will cause embrittlement of the weld and heat affected zone. Since these duplex stainless steels are mostly used for large-scale components, post-weld heat treatment is not possible. The welds and heat-affected zones should have all the required chemical compositions and mechanical properties directly after welding. Tests have shown that when the ferrite content in the metal structure of the weld joint and heat-affected zone is greater than 80%, the toughness is increased and the crack generation is increased. Therefore, the chemical composition of the weld, particularly the mass fraction of Ni, and the cooling rate are controlled to prevent the tendency of single-phase ferrite and coarse grains and the occurrence of cracks.


First, the cooling rate must be kept within a suitable range so that the molten metal has enough time to generate enough austenite. The cooling rate is determined by the input heat, preheating temperature, inter-pass temperature and the thickness of the base metal. Especially when welding large workpieces, the cooling rate of the heat-affected zone of the base metal is low, which will prolong the residence time of the temperature zone causing embrittlement. The residence time of the base metal in the high-temperature zone is also prolonged in the near seam zone, which is not conducive to the control of the weld pool and the welding of the roots. Therefore, pay attention to the use of small welding heat energy, the inter-pass temperature is not better than 150>, so that the weld metal has a reasonable ratio of ferrite-austenite double.

 

Secondly, by appropriately increasing the mass fraction of Ni, Mn, and N in the electrode or welding wire, austenite formation can be promoted. This is a simple and easy method for the production site where the cooling speed is not easy to control. In addition, the conductivity of the electrode varies with the temperature, so the welding device with a smaller diameter is used. The device itself is independent and can meet the requirements for long tube welding. For small weldments, simply remove the active pulley and use a three-jaw self-centering chuck to hold the workpiece.


The two complement each other, increasing flexibility and expanding the range of applications for the equipment.


Easy to use and operate. A pair of rollers is equivalent to a V-shaped iron, and the placement of the workpiece can automatically complete alignment positioning without the need for other operations such as locking.
Four pairs of rollers are installed on the optical axis (it can be increased or decreased as needed). It is not necessary to add additional centering fixtures to realize the butt welding between workpieces of the same diameter, and it can ensure sufficient centering accuracy.


With angle-line speed automatic conversion function. The normal welding process parameters are given by the line speed of the weld formation. The device has no relative sliding between the workpiece and the roller. The linear velocity of the workpiece is the linear velocity of the outer diameter of the roller. When the diameter of the workpiece changes, as long as the rotation speed of the roller is not changed, the angular velocity of the workpiece can be automatically changed so that the line speed remains unchanged. This is very different from using a three-jaw self-centering chuck each time the linear speed is converted based on the fixture rotation speed (that is, the workpiece angular velocity) and the workpiece diameter. If the corresponding relationship between the linear speed of the roller and the rotation speed adjustment handle is calibrated, the diameter of the workpiece will not be limited in the subsequent use, and the line speed can be adjusted by directly pressing the calibration value, which will bring about a very good use. Great convenience.


4 Conclusion The device has achieved good results after practical application, and the welding quality has met the design requirements, thus solving the problems in production, with little investment, in exchange for the improvement of the processing capacity of existing equipment. For some other similar structures, but the size of the workpiece is different, the basic structure of the device can be used, only the appropriate adjustment of the optical axis and the size of the roller can be welded to these workpieces.